Printed circuit boards (PCBs) are widely used electronic substrates in the electronics and telecommunications industry. PCBs generally include one or more layers of an insulating substrate (e.g., plastic) on which an electrical circuit is formed by depositing a predetermined pattern of a conducting metal (e.g., copper) for connecting various electronic components (e.g., semiconductors) which are mounted on or etched in the PCB layers. Many of these electrical circuits include components which operate at a high radio frequency (RF). The RF emissions from these components can interfere with the proper operation of other components or circuits in the vicinity of the PCB. Thus, it may be important to block or shield these RF emissions in order to prevent RF interference.
Conventionally, a metal "shield can," generally having the shape of an inverted open box, can be utilized to cover an electronic component or a specific portion of circuitry in or on a PCB which is causing RF interference. Shield cans are usually applied to the appropriate section of a PCB either by soldering or forced galvanic engagement. For proper and effective RF shielding, it is conventionally desired that a shield can uniformly contact a PCB. There may be little or no shielding of RF energy if gaps exist between the shield can and the PCB.
A conventional shield can 10, illustrated in FIG. 1A, includes a cover 12 and a plurality of side walls 14 formed from sheet metal. For rigidity, a lip 16 is formed around an upper portion 14a of each wall and a pair of cross members 18a, 18b are provided, as illustrated. The illustrated shield can 10 also includes a "pick-up-point" 17 located at the intersection of the cross members 18a, 18b that facilitates the placement of the side walls 14 by surface mount equipment. The cover 12 is attached at a later point in the manufacturing process.
To mount the illustrated shield can 10 on a PCB 11, a lower end portion 14b of each wall 14 is placed into a solder paste deposit 20 on a mounting pad 22, as illustrated in FIG. 1B. Heat is then applied to cause the solder paste 20 to reflow which produces the illustrated solder joints (also referred to as "fillets") 24 in FIG. 1C. When cooled, each fillet 24 can provide a bond between a mounting pad 22 and a respective wall 14. The cover 12 is configured to be secured to the upper portions 14a of the walls 14 to enclose a portion of a PCB. Unfortunately, the lip 16 and cross members 18a, 18b can hinder inspection and re-work performed on soldered components under these features, and/or removal of the shield can 10 at a later time.
For a shield can wall formed from sheet metal to be properly soldered to a PCB, it is typically important that the sheet metal be substantially flat, both initially when the side wall is formed, and during reflow operations. Unfortunately, heat from reflow operations may cause sheet metal to warp. This may result in gaps between a shield can wall and a PCB which may threaten the effectiveness of the shield can in blocking RF emissions. The presence of gaps may require costly re-work to properly solder a wall to a PCB.
A conventional molded shield can 10' that is often preferred over shield cans formed from sheet metal is illustrated in FIG. 2A. The illustrated molded shield can 10' includes a cover 12' and a plurality of molded side walls 14', and a pickup point 17' for surface mount equipment. As illustrated in FIG. 2B, each side wall 14' of the molded shield can 10' has a thickness T.sub.2 greater than a thickness T.sub.1 of the sheet metal walls 14 of the shield can 10 illustrated in FIGS. 1A-1C (i.e., T.sub.2 &gt;T.sub.1).
To mount the molded shield can 10' of FIG. 2A on a PCB, a lower end portion 14b' of each wall 14' is placed within solder paste 20 on a mounting pad 22, as illustrated in FIG. 2B. Heat is then applied to cause the solder paste 20 to reflow which produces the illustrated solder fillets 24 in FIG. 2C. When cooled, each fillet 24 can provide a bond between a mounting pad 22 and a respective wall 14' . The cover 12' is configured to be secured to the upper portions 14a' of the walls 14' to enclose a portion of a PCB 11.
By increasing the thickness of each wall 14', the need for a lip and cross members to provide rigidity can be eliminated. As a result, any additional re-work can be performed without hindrance therefrom. Molded shield cans are also preferred because a mold can be cost-effectively prepared for complex shapes and configurations that may be otherwise difficult into which to form sheet metal.
Unfortunately, molded shield cans having thicker walls may utilize significantly more PCB space than shield cans with walls formed from sheet metal. For electronic devices, such as radiotelephones and other communication devices where PCB space is limited, molded shield cans having walls thicker than sheet metal may be impractical or may compromise design/component spacing.